-- bind the tunnel to the device NAME so that
tunneled packets will only be routed via this device and will
not be able to escape to another device when the route to endpoint changes.

nopmtudisc

-- disable Path MTU Discovery on this tunnel.
It is enabled by default. Note that a fixed ttl is incompatible
with this option: tunnelling with a fixed ttl always makes pmtu discovery.

key K, ikey K, okey K

-- (only GRE tunnels) use keyed GRE with key K. K is
either a number or an IP address-like dotted quad.
The key parameter sets the key to use in both directions.
The ikey and okey parameters set different keys for input and output.

csum, icsum, ocsum

-- (only GRE tunnels) generate/require checksums for tunneled packets.
The ocsum flag calculates checksums for outgoing packets.
The icsum flag requires that all input packets have the correct
checksum. The csum flag is equivalent to the combination
``icsumocsum''.

seq, iseq, oseq

-- (only GRE tunnels) serialize packets.
The oseq flag enables sequencing of outgoing packets.
The iseq flag requires that all input packets are serialized.
The seq flag is equivalent to the combination ``iseqoseq''.

1cm NB.
I think this option does not
work. At least, I did not test it, did not debug it and
do not even understand how it is supposed to work or for what
purpose Cisco planned to use it. Do not use it.